The present invention relates in general to semiconductor technology, and more particularly to structures and methods for forming high aspect ratio trenches. Merely by way of example, the invention has been applied in a shielded gate trench field effect transistor (FET). But it would be recognized that the invention has a much broader range of applicability.
Shielded gate trench FETs and trench gate FETs are widely used in power electronics. In a shielded gate trench FET, the shield electrode reduces the gate-drain capacitance (Cgd) and improves the breakdown voltage of the transistor without sacrificing the transistor on-resistance. FIG. 1 is a simplified cross sectional view diagram of a conventional shielded gate trench MOSFET. An n-type epitaxial layer 102 extends over n+ substrate 101. N+ source regions 108 and p+ heavy body regions 106 are formed in a p-type body region 104 which is in turn formed in epitaxial layer 102. Trench 110 extends through body region 104 and terminates in the drift region which is the portion of epitaxial layer 102 extending between body region 104 and substrate 100. Trench 110 includes a shield electrode 114 below a gate electrode 122. Gate electrode 122 is insulated from its adjacent silicon regions by gate dielectric 120, and shield electrode 114 is insulated from its adjacent silicon regions by a shield dielectric 112 which is thicker than gate dielectric 120. The gate and shield electrodes are insulated from each other by a dielectric layer 116 also referred to as inter-electrode dielectric or IED.
For many applications a key performance characteristic of the trench FET is its switching speed. To maximize the switching speed of the trench FET it is desirable to minimize the resistivity of its gate material. As shown in FIG. 1, both the shield electrode 114 and the gate electrode 122 are formed inside trench 110. With the advancement to technology, device size continues to shrink, and the aspect ratio of trench structures and recesses continues to increase. As a result, conventional methods for forming trench structures and recesses in general suffer many limitations. Some of the limitations are illustrated in FIGS. 2A and 2B and described in more detail further below.
Thus, there is a need for simple and cost effective techniques for filling high aspect ratio trenches and recesses in a void-free manner.